A spark gap device is one in which a discharge occurs between two electrodes. The device may be used as a switch, in which case a trigger electrode is included and the discharge is triggered by applying a suitable potential to it. A spark-gap device may alternatively be of the over-voltage type, in which breakdown occurs when a threshold potential difference between the two electrodes is exceeded. In presently known spark gap devices, the electrodes are rod-like and contained within a cylindrical envelope.